1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image is formed on an image bearing member and an image is formed on a transfer material by transferring the toner image onto the transfer material.
2. Description of the Related Art
For a four-color full color image forming apparatus, an image forming apparatus including four (four colors) image forming portions which form toner images having different colors respectively is known. The four image forming portions are disposed from an upstream side to a downstream side along an intermediate transfer belt moving direction, the toner images having the different colors formed by the image forming portions are sequentially primary-transferred onto the intermediate transfer belt, and the four color toner images are superposed on the intermediate transfer belt. Then, the four color toner images on the intermediate transfer belt are collectively secondary-transferred onto a transfer material. Then, the four-color full color toner image is fixed on the transfer material by heating the four color toner images with a fixing device.
Application stop timing of a development bias voltage and a charging bias voltage in ending a recording operation (image forming operation) will be described below.
In each image forming portion, the charging performed by the charger driven during the image formation is ended in ending the recording operation. A development nip region is formed in a region where a surface of a photosensitive drum is in contact with a developer retained by a development sleeve of a development device. On the photosensitive drum surface, immediately after a rear end of a region (hereinafter referred to as “charged portion”) which is charged by the image formation reaches the development nip region, the photosensitive drum surface close to the development sleeve becomes 0 V.
Accordingly, in the case where the development bias voltage (for example, −550 V) of the development sleeve is turned off after the rear end of the charged portion on the photosensitive drum passes through the development nip region, a large contrast potential Vcont (=|0−(−550)|=550 V) is formed in the whole region of the photosensitive drum surface passing through the development nip region, and the toner in the developer adheres to a non-image portion before the development is ended. As a result, a band-like toner image is formed at the rear end of the charged portion in ending the recording operation.
On the other hand, in order to prevent the adhesion of the toner to the non-image portion, it is necessary to turn off the development bias voltage of the development sleeve before the rear end of the charged portion in the photosensitive drum surface charged to a surface potential Vd (for example, −700 V) reaches the development nip region. However, under such circumstances, a part of the photosensitive drum surface charged to the surface potential Vd (−700 V) passes through the development nip region after the development bias voltage is turned off. At this point, a large fog removal potential Vback (=|−700−0|=700 V) is formed between the development sleeve and the photosensitive drum surface. Therefore, in the case where a two-component developer including the toner and a carrier is used as the developer, there is generated a problem in that the carrier in the developer adheres to the photosensitive drum surface.
In order to cause both the toner and the carrier not to adhere to the photosensitive drum surface, it is necessary that the development bias voltage be turned off at the same time the rear end of the charged portion on the photosensitive drum reaches the development nip region. In consideration of the actual various fluctuations, it is difficult to completely synchronize the turn-off timing in each time. Furthermore, because the development nip region has a predetermined width, it is difficult to synchronize the turn-off timing.
Accordingly, either the carrier or the toner on the development sleeve adheres to the photosensitive drum. Particularly when the carrier of the development sleeve jumps to the photosensitive drum, a cleaning blade of a cleaning device for cleaning the photosensitive drum surface causes the carrier to damage the photosensitive drum surface and to shorten the life of the photosensitive drum.
Therefore, conventionally the carrier adhesion is completely prevented by turning off the bias voltage applied to the development sleeve immediately after the rear end of the photosensitive drum surface charged to surface potential Vd (−700 V) by the charger reaches the development nip region, which forms the band-like toner image at the rear end of the charged portion in ending the recording operation.
Unlike the carrier, the cleaning blade hardly causes the toner to damage the photosensitive drum surface. In the conventional configuration, although a problem of increasing toner consumption is generated, the good image can be maintained for a long period while the adhesion of the carrier to the photosensitive drum is not increased (for example, see Japanese Patent Application Laid-Open No. 2003-280483).
On the other hand, because the same problem is generated in starting the recording operation, the development bias voltage applied to the development sleeve is turned on immediately before a leading end of the charged portion in the photosensitive drum surface charged to the surface potential Vd (−700 V) by the charger reaches the development nip region. However, in starting the recording operation, the toner in the development nip region is discharged onto the photosensitive drum in ending the recording operation, and only a trace of the toner remains in the development nip region. Therefore, the formation of the band-like toner image becomes minor compared with the band-like toner image in ending the recording operation.
As described above, in ending the recording operation, when the above configuration is used as allocation stop timing of the development bias voltage and charging bias voltage, the band-like toner image is formed at the rear end of the charged portion although the carrier does not adhere to the photosensitive drum.
The band-like toner image is transferred tentatively to the intermediate transfer belt by a primary transfer roller, and the band-like toner image is removed and recovered by an intermediate transfer belt cleaning member located on the downstream side of a secondary transfer roller. Otherwise, the band-like toner image remains on the intermediate transfer belt, which causes in-machine contamination. Therefore, it is necessary to continuously rotate the intermediate transfer belt until the band-like toner image is conveyed to the intermediate transfer belt cleaning member and removed and recovered. However, there is generated a problem in that a long time is required for the rotation of the intermediate transfer belt in ending the recording operation.
Because the transfer material to be transferred does not exist in the band-like toner image when the band-like toner image is conveyed to the secondary transfer roller located on the upstream side of the intermediate transfer belt cleaning member, a part of the band-like toner image is transferred to the secondary transfer roller, which causes the contamination of the secondary transfer roller. In order to solve the above problems, a secondary transfer roller cleaner is provided to remove the contamination of the secondary transfer roller, and the contamination of the secondary transfer roller is decreased. Then, the band-like toner image is conveyed to the intermediate transfer belt cleaning member, and the intermediate transfer belt is continuously rotated to prevent the contamination in the machine until the band-like toner image is removed and recovered.
When the four-color full color image forming apparatus has the configuration of the bias stop timing in ending the recording operation, the band-like toner images are formed according to the bias stop timing of each color. The band-like toner images are transferred to the intermediate transfer belt as well as the four color toner images primary-transferred to the intermediate transfer belt by electric action and pressing force of the primary transfer roller.
Accordingly, in the four-color full color image forming apparatus, the four color band-like toner images contaminates the secondary transfer roller. Additionally, in the four color band-like toner images formed according to the bias stop timing of the colors, it is necessary to continuously rotate the intermediate transfer belt until the band-like toner image located on the uppermost stream side of the intermediate transfer belt is conveyed to the intermediate transfer belt cleaning member and removed and recovered. As a result, there is generated the problem in that a large amount of time is required for the rotation of the intermediate transfer belt in ending the recording operation.
Furthermore, as described above, in the band-like toner image, because the development bias voltage Vdc (550 V) is applied to the development sleeve when the photosensitive drum has the surface potential of 0 V, a development process is performed by the extremely high contrast potential Vcont (−550 V). Usually, from the standpoint of safety, a solid image portion on the photosensitive drum has a surface potential V1 smaller than the value of 0 V (for example, −200 V). As a result, compared with the contrast potential Vcont (=|V1−Vdc|=|−200+550|=350 V) in the usual solid image formation, a large amount of toner is placed on the band-like toner image formed at the rear end of the charged portion in ending the recording operation.
That is, in the four-color full color image forming apparatus, the four color band-like toner images which have the toner amounts larger than the toner amount corresponding to the solid image respectively are formed on the intermediate transfer belt in ending the recording operation.
In order to prevent the in-machine contamination, it is necessary that all the four color band-like toner images on the intermediate transfer belt be conveyed to the intermediate transfer belt cleaning member. However, when the band-like toner image is conveyed to the secondary transfer portion located in the course of conveyance, the band-like toner image comes into direct contact with the secondary transfer roller, and a part of the band-like toner image is transferred to the secondary transfer roller to contaminate the secondary transfer roller by the electric action and pressing force.
The contamination can be removed by the secondary transfer roller cleaner. However, sometimes the secondary transfer roller cleaner cannot remove the four color band-like toner images on which the extremely large amount of toner is placed by one-time cleaning. When a backside of the transfer material is brought into contact with the contaminated part in the next process, there is generated a problem in that the backside of the transfer material is contaminated. Sometimes the toner is hardly removed depending on a material of the secondary transfer roller. For example, the toner is hardly removed when a surface shape of the secondary transfer roller is roughened.
On the other hand, there is a method in which the secondary transfer roller is rotated more than once to completely clean the secondary transfer roller while the image is not formed and the transfer material is fed to resume the image formation. However, efficiency is reduced in the above method, because the image cannot be formed until the secondary transfer roller is sufficiently cleaned.